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Process: Timer Blocks Zip File ©Christopher Leigh

Here are four teaching and learning handouts in Word format which may help you in using these system modules. 2c Using System Modules, 5a Electronic Switches, 6b System Module Capacitors, and 7b System Module Astables. Check in the Handouts page for more useful handouts for teaching and learning.

555 Monostable

The 555 timer chip can be used for all sorts of circuits. Here it acts as a monostable. A low input pulse will switch the output high. This will go low again after a certain time. The time is given by the formula 1.1 x R x C, where R is in Ohms and C in Farads. It is usually more convenient to have R in mega-ohms and C in micro-farads. So 1M and 10uF will give an on-time of 11 seconds. See Doctronic’s Beastie Zone for more details. See also the Latch Blocks page on this site.

Logic Monostable - High Pulse

This block requires an input block that goes high. This makes the output high for a time dependent on the size of the capacitor and (variable) resistor. The output stays high even if the input goes high again. If the input is pulsed a second time while the output is high, then the timing begins again. Instead of the Schmitt Not gates (40106), you could use Nand gates, as long as they are Schmitt versions (4093).

Logic Monostable - Low Pulse

This block requires an input block that goes low. This makes the output low for a time dependent on the size of the capacitor and (variable) resistor. The output stays low even if the input goes low again. If the input is pulsed a second time while the output is low, then the timing begins again. Instead of the Schmitt Not gates (40106), you could use Nand gates, as long as they are Schmitt versions (4093).

Touch Timer

With a touch pad and a capacitor you can build a timer block. It needs to feed into a FET driver with a high input resistor, say 10M, from the FET’s gate to zero volts. The FET will be switched on as long as the voltage across the capacitor is above the gate threshold voltage for the FET. The capacitor discharges through the gate resistor (10M) providing a fairly long time period. Exact time period depends on the FET’s threshold voltage and the capacitor’s leakage current.

Buffered Touch Timer

The block above is slow to charge up due to the relatively high resistance of the touch pad when touched. A smaller capacitor can charge up more quickly, turning the buffer transistor on allowing the main capacitor to charge up quickly. This means that even a brief touch will charge the capacitor fully providing the full time delay. For other touch circuits see the touch sensor page.